Overlay coatings on plain journal bearings are well known. Such coatings are used to improve the running characteristics of plain bearings. Generally, overlay coatings are relatively soft metal alloys having a hardness in the region of about 15 Hv; are frequently based on alloys of lead; and, are deposited on another harder bearing alloy at a thickness in the range from about 10 to 30 μm. Overlay alloys of the type under consideration are usually applied by electro-deposition from aqueous plating solutions.
The bearings on which the overlays are deposited are of generally cylindrical or, more commonly, semi-cylindrical form as half-bearing shells which support the crankshaft journals of internal combustion engines, for example. Such bearings generally comprise a layer of a strong backing material such as steel, for example, on which is bonded a layer of a bearing material frequently chosen from alloys of aluminium or copper. The method of attaching the layer of bearing alloy to the strong backing may be any that is suitable and may include techniques such as pressure welding of sheets of bearing alloy to the backing; the casting of molten alloy onto the backing; or, the sintering of powders of alloy to the backing, for example, these methods not being exhaustive. The overlay alloy coating is deposited on the surface of the harder bearing alloy and endows the finished bearing so formed with properties which include conformability and the ability to embed dirt particles and so prevent scoring of a shaft journal by particles of debris carried in the lubricating oil. Although overlay alloys in their bulk form are relatively weak alloys, they have the ability when applied as a thin layer to another, harder bearing alloy to increase the fatigue strength of a bearing embodying that harder and intrinsically stronger bearing alloy. This is effected due to the conformability of the overlay alloy by being able to deform slightly to accommodate slight mis-alignments, especially in new engines during the “running in” phase, and so spread the load more evenly across the bearing surface area.
As noted above, many conventional overlay alloys are based on alloys of lead. Lead is a toxic metal which will eventually be phased out of use by governmental legislation throughout the world. In order to make the lead-based overlay layer less prone to corrosion in hot engine oils about 10 weight % of tin is frequently added or, alternatively, 7 to 10 wt % of indium. Indium, however, is relatively very expensive compared with tin and tends to be used for more expensive, higher performance vehicles. However, when tin is used in the overlay alloy and is deposited upon a harder bearing alloy such as copper-lead, for example, a problem exists in that the tin under engine operating conditions tends to diffuse out of the overlay into the lead of the underlying bearing alloy, as does indium. This is solved by coating the surface of the underlying, harder bearing alloy with a thin diffusion barrier of about 1–3 μm of a metal such as nickel. However, this is not entirely satisfactory as diffusion still occurs and the overlay still becomes depleted in tin due to the formation of non-equilibrium intermetallic compounds such as Ni3Sn or Ni3Sn2 which are not good bearing materials in the situation where the shaft journal wears through the overlay to the underlying interface comprising these intermetallic compounds.
With the ever increasing demands placed on bearings by engines having higher specific outputs and operating at higher engine revolutions, there has been a demand for these relatively soft overlay alloys to have improved wear resistance whilst at least maintaining existing levels of fatigue, cavitation resistance and corrosion resistance. This demand has resulted in the development of so-called lead-tin-copper overlay alloys an example of which is Pb-10Sn-2Cu.
Thus, it is an object of the present invention to provide an overlay layer which is not toxic and a further object is to provide an overlay which does not form undesirable compounds at an interface with an underlying, harder bearing material. A yet further object is to provide an overlay having improved performance over known lead-based overlay alloys.
According to a first aspect of the present invention there is provided a plain bearing having an overlay material layer at a sliding surface of the plain bearing, the plain bearing comprising a layer of a strong backing material, a layer of a first bearing alloy bonded to the strong backing material and a layer of a second bearing alloy comprising said overlay material bonded to said first bearing alloy layer wherein said second bearing material comprises tin having included in the matrix thereof an organic levelling agent.
The tin overlay layer according to the present invention comprises essentially pure tin in that there are no metallic alloying constituents, other than unavoidable impurities, however, the tin is deposited from a bath containing additions of one or more organic materials which have the effect of so-called “levelling” on the electro-deposited tin layer.
Organic materials which have been tested in bearings of the present invention embodying tin overlays include nonylphenolpolyglycolether and pyrocatechol. The content of the organic material in the plating bath has an influence on the degree of levelling achieved in the deposited tin layer, the degree of levelling being reflected in the surface roughness of the tin layer.
At low levels of organic levelling agent, too low for the full benefit of the present invention to be felt, the surface appearance of the bearing surface is one of a generally crystalline appearance having pools of smooth material distributed over the surface. At a content of organic levelling agent where the whole surface is smooth, this is the desirable minimum content.
It is believed that the organic levelling agent is incorporated in the matrix of the deposited tin layer as polymer chains occluded in the matrix structure such as in the form of an organo-metallic tin compound, for example. The polymer chains appear to impart a preferred orientation to the tin atoms during deposition which has been found to give improved slip properties. Improved slip properties have been evidenced by lower coefficients of friction in the tin layer compared with ordinary tin deposits without the levelling additions. The surface of the tin overlay of the bearing of the present invention is very smooth giving a lower degree of friction against a co-operating shaft journal which in turn gives improved compatibility between bearing surface and shaft journal resulting in lower wear rates.
The organic constituent of the tin overlay produces an increased hardness in the range from about 20 to 30 Hv. Pure tin with no organic levelling agent, depending upon its condition, has a hardness of about 8–12 Hv. The hardness of the tin overlay can be changed depending upon the content of the organic levelling agent in the plating bath; the lower the content, the lower the corresponding hardness. The reverse is also true in that as the content of levelling agent increases, so also does the hardness. However, it is possible to have too high a content of organic levelling agent such that the hardness is too high and high internal stresses are produced in the deposit which can lead to cracking of the tin deposit. It is intended that the overlay of the bearing of the present invention operates in a similar manner to conventional overlays in that the overlay layer is sufficiently soft to permit particles of dirt circulating in the lubricating oil to become embedded in the overlay so as to prevent such dirt particles from scoring the shaft journal. Whilst the tin overlay of the present invention is harder than pure tin by a factor of X2 to X3 it is still sufficiently soft to provide the required characteristic of dirt embeddability thus, the preferred hardness range is 20 to 30 Hv.
The bearing of the present invention may preferably have an interlayer between the surface of the first bearing material and the tin overlay to act as a diffusion barrier therebetween. The metal layer may be of a thickness lying in the range from about 0.1 to about 3 μm with a thickness of 1 to 2 μm being preferred, however, the actual thickness is of comparatively little importance in terms of bearing performance. The metal may be selected from the non-exhaustive group including nickel, cobalt, copper, silver, iron and alloys of these metals, for example. It has been found that under engine operating conditions the tin overlay reacts with the nickel interlayer over time to form the stable equilibrium intermetallic compound, Ni3Sn4, due to the presence of effectively an excess of tin. As noted above, prior art lead-10tin overlays tended to form the unstable, non-equilibrium Ni3Sn or Ni3Sn2 compounds which are poor bearing materials and have inferior compatibility with a shaft journal and have been blamed in the past for causing seizure when the overlay has worn through to the interlayer. Ni3Sn4 on the other hand is a very good bearing material and thus, the overlay of the present invention in addition to having superior resistance to wear and cavitation erosion is also less prone to seizure when the overlay is nearing the end of its life. Thus, this unforeseen effect of generating a good bearing material at the interface is seen as a significant advantage of the bearing of the present invention.
As with known overlay layers, the thickness of the overlay of the bearing of the present invention may lie in the range from about 10 to 30 μm with 13 to 18 μm being preferred.
The deposition conditions for tin overlays according to the present invention may be varied to produce a range of microstructures. For example, analysis of the tin overlay layer by SEM has revealed no discernible grain size; even at magnifications of X5000 and X10000 no grains can be resolved. However, coatings having grain sizes of up to 3 μm may be produced. It is preferred, however, that a smaller grain size is produced as these provide improved bearing properties.
According to a second aspect of the present invention, there is provided a method for the deposition of an overlay layer onto the surface of a plain bearing, the bearing comprising a strong backing material having a layer of a first bearing material thereon, said overlay being deposited upon the surface of said first bearing material, the method comprising the steps of: providing a bearing having a surface on which to deposit said overlay; immersing said bearing in a plating solution having a supply of tin ions and an organic levelling agent in said solution; making said bearing cathodic with respect to an anode in said solution; and depositing an overlay of tin, apart from unavoidable impurities, said tin overlay also having said organic levelling agent included in a matrix thereof.
It is preferred to deposit the tin overlay of the bearing of the present invention by using a so-called “slot jig” wherein the bearing is held with its joint faces against a back face of the slot jig with the bore of the bearing facing the slot, the bearing axis and slot being generally parallel to each other. The plating solution, in which the bearing and slot jig are immersed, is also then sparged through the slot towards the bearing bore.
In this way it has been found that relatively high current densities of 2 to 3 A/dm2 may be employed compared with less than 1 A/dm2 where the bearing is merely immersed in the plating solution without sparging thereof. Furthermore, the quality of the deposited tin layer is greatly improved compared with that produced without sparging. The use of high current density permitted by the slot jig and sparging technique also reduces plating time from more than 40 minutes to less than 20 minutes.
A typical plating solution producing a tin/organic material overlay on a bearing according to the present invention may have a composition as follows:
Sn++32–38g/lSnSO458–68g/lH2SO4185–210g/lCu<50mg/lChloride<20ppm
Levelling agent additions of nonylphenolpolyglycolether (10–25%) in a methanol carrier (2.5–10%) in the range from 18 to 70 ml/l to the solution specified above have been tested. At the lower end of the range it was found that the degree of levelling and hardness increase was insufficient whilst at the upper end of the range it was found that there was too much inherent stress in the tin deposit and cracking occurred. It was found that concentration in the range from 25 to 55 ml/l gave useful increases in overlay performance with little or acceptable deterioration of the fundamental requirements of an overlay alloy in terms of conformability and dirt embeddability. The content of pyrocatecol was 2.5–10% and amphoteres tensid 2.5% maximum.